Electrochemical heat treatment

ABSTRACT

A method and apparatus for heat treating metal wire or strip, for example for annealing steel wire or strip, in which the metal and an aqueous electrolyte solution are moved continuously relative to one another. The metal is cathodically polarized with respect to an anode in the electrolyte. When the current density and interelectrode potential difference are sufficiently high, a substantially stable sheath of hydrogen forms around the metal to separate the metal from the electrolyte and an electrical discharge takes place across the sheath. The metal within the sheath is rapidly heated by a mechanism that is not fully understood but is referred to as electrochemical heating. The electrolyte is preferably directed in a flow at the wire or strip from one or more nozzles, and the wire or strip drawn through the flow.

United States Patent [72] lnventors 2,932,502 4/1960 Rudd et a1.

Appl. No.

Filed Patented Assignee Priority ELECTROCHEMICAL HEAT TREATMENT 7 Claims, 11 Drawing Figs.

U.S. Cl

Int. Cl Field of Search References Cited UNITED STATES PATENTS 219/71, 148/154, 148/156, 148/150 C2ld 9/52 204/140; 148/150; 219/71 Primary ExaminerRobert K. Mihalek Attorney-Sghrue, Rothwell, Mion, Zinn & MacPeak ABSTRACT: A method and apparatus for heat treating metal wire or strip, for example for annealing steel wire or strip, in which the metal and an aqueous electrolyte solution are moved continuously relative to one another. The metal is cathodically polarized with respect to an anode in the electrolyte. When the current density and interelectrode potential difference are sufficiently high, a substantially stable sheath of hydrogen forms around the metal to separate the metal from the electrolyte and an electrical discharge takes place across the sheath. The metal within the sheath is rapidly heated by a mechanism that is not fully understood but is referred to as electrochemical heating. The electrolyte is preferably directed in a flow at the wire or strip from one or more nozzles, and the wire or strip drawn through the flow.

' PATENTED M] 9m sum 1 or 4 F I 6. 2A.

ELECTROCHEMICAL HEAT TREATMENT This invention relates to heat treatment and in particular to a method and apparatus for the heat treatment of elongate conducting materials, such as metal, for example in the form of wire or strip. One application of the invention is to the continuous annealing of steel wire or strip. The word wire" is used to mean material of substantially circular cross section, but also includes wires made up from a number of parallel or twisted strands of material. The phrase elongate material" includes wire, strip, rod and tube material.

According to the present invention in one aspect there is provided a method of heat treating elongate conducting material comprising causing the material to be treated and an aqueous electrolyte solution to move continuously relative to one another, the material being cathodically polarized with respect to an anode associated with the electrolyte at a current density and to a potential sufficient to cause a substantially stable gaseous sheath to form between the electrolyte and the material and an electrical discharge to take place across the sheath, whereby the material is rapidly heated.

According to the invention in another aspect there is provided apparatus for the heat treatment of elongate conducting material comprising an arrangement for moving aqueous electrolyte solution and the material to be treated relative to one another, and an anode associated with the electrolyte, whereby when the material is cathodically polarized with respect to said anode at a current density and to a potential sufficient to cause a substantially stable gaseous sheath to form between the electrolyte and the material and an electrical discharge to take place across the sheath, the material is rapidly heated.

Preferably a flow of the electrolyte is directed at the material, suitably from one or more nozzles which may also be adapted to serve as the anode.

Detailed examination of the elongate material during the heating, which may be termed electrochemical heating, reveals the that the mechanism involved is complex. It appears that the cathode surface, i.e. the surface of the material, becomes enveloped by a sheath of hydrogen across which a discharge takes place and the wire is heated by hydrogen ion bombardment. This discharge is probably partly a glow discharge but there is experimental evidence which suggests that a large number of very small arcs are continually occuring and being extinguished across the sheath. The latter would imply that the sheath is never truly continuous or stable at any particular point. However it is readily established by experiment that under suitable conditions a gaseous sheath may be caused to form which appears to the eye to be substantially continuous and substantially stable. Moreover under these conditions the elongate material is substantially uniformly heated, The phrase substantially stable is therefore taken to mean that the sheath appears stable to the eye, thought it may not be to microscopic examination, and the presence of a stable sheath may be verified by examination of the heat treated material for uniformity. Under incorrect conditions, when the sheath is unstable, bubbles of gas may grow and collapse adjacent to the material, so causing fluctuations in the current flow in the discharge which cause nonuniform heating or the material.

The current density and the potential necessary to initiate formation of the sheath are dependent on a number of variables, in particular upon the electrode spacing, the particular electrolyte used, the concentration and temperature of the electrolyte and to some extent on the speed of relative movement between the electrolyte and the material. The current density and potential necessary are also to some extent interdependent, and also, once the sheath has been formed, the current and potential may change or be changed without affecting stability of the sheath. Below certain current densities and voltages it may be found that reduction in the relative speed of movement to a low value detrimentally affects stability of the sheath.

Similarly the temperature attained by the material and the length of time taken to attain a given temperature are dependent on the cross-sectional dimensions of the material, its thermal conductivity, and the speed of the relative movement, as well as the values of the applied voltage and current density. The length of time may be shortened by increasing the temperature or strength of the electrolyte, or by increasing the applied potential. 1

The cfficiency of the heating also depends on a number of variables. Under certain conditions about 30 percent of the applied electrical energy appears as heat in the material, the remainder serving to heat the electrolyte. The efficiency appears to increase slightly upon increasing the current density, upon decreasing the mass flow rate of the electrolyte relative to the material, upon increasing the speed of the material relative to the electrolyte, and/or upon reducing the spacing between the anode and the cathode. The electrolytes of higher conductivity also appear to enhance efficiency. The mass flow rate should not be decreased too far, since nonuniform heating can occur at low flow rates.

It is very desirable that the area of the anodesurface in contact with the electrolyte be at least twice as great as the corresponding surface of the cathode, i.e. the material. This is because gas is liberated at both electrodes hydrogen at the cathodes and oxygen at the anode, and if the anode area is small then an increased applied potential will be necessary to break down the gaseous sheath on both the electrodes. By making the anode area large, the quantity of oxygen liberated is spread over a wide area and does not affect the heating process at the cathode.

The maximum crosssectional dimensions of the elongate material that can uniformly be heat treated with practical efficiency depends on the order of the maximum temperature required and the conductivity of the material.

In order to illustrate the effects discussed above, several particular examples will be given below, but firstly embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of one embodiment of electrochemical heat treatment apparatus according to the invention, having a submerged nozzle,

FIG. 2A illustrates a second embodiment having a free nozzle,

FIG, 28 illustrates a modification of the nozzle of FIG. 2A,

FIG. 3A l illustrates a third embodiment having two nozzles,

FIG. 33 illustrates a fourth embodiment having an annular nozzle,

FIG. 4A illustrates a fifth embodiment having a plurality of nozzles,

FIG. 48 illustrates a modification of the arrangement of FIG. 4A,

FIG. 4C illustrates a sixth embodiment wherein strip material is drawn over a drum,

FIG. 5 illustrates a seventh embodiment, for selective treatment,

FIG. 6 illustrates an eighth embodiment having a broad nozzle, and

FIG. 7 illustrates a ninth embodiment wherein wire is drawn through a bath.

Referring now to FIG. 1 there is shown electrochemical heat treatment apparatus comprising a bath 10 divided by a weir 11 of adjustable height. A suitable electrolyte l2 is pumped from the right-hand part of the bath 10 by pump 13 through a conduit 14 to a nozzle 15 which is submerged in the electrolyte in the left hand part of the bath 10. The nozzle 15 has a vertically disposed rectangular slit orifice 16. A metal wire 17 to be treated is drawn from a stock reel upwardly through a seal 18 in the floor of the bath to pass closely adjacent and aligned with the slit orifice 16, over a jockey wheel 19 to a winding spool. Suitable electrical connections to the nozzle 15 and jockeywheel 19 serve anodically to polarize the nozzle with respect to the wire 17. In operation the pump 13 forces electrolyte through the orifice 16 to pass a flow or stream of electrolyte over the moving wire.

FIG. 2A shows an a embodiment wherein the metal wire 17 to be treated is moved horizontally through a vertically downwardlymoving stream of electrolyte issuing from the nozzle 15 mounted in the atmosphere, or preferably in an enclosed local atmosphere above a collecting and recirculating bath.

FIG. 2B shows a modification of the nozzle of FIG. 2A wherein the nozzle 15 is provided with opposed downward side extensions 20 which confine the electrolyte flow to substantially surround the wire, thus minimizing dry spots behind the wire. In this way the anode and cathode are also closely spaced. thus effectively reducing the overall resistance across the electrolyte and slightly increasing the efficiency of the process. This modification has been found to be particularly suitable for the heat treatment of elongate material in the form of wire.

FIG. 3A shows an arrangement of two nozzles 30 having circular orifices disposed at an angle and spaced about the circurnference of a wire 31 moving upwardly through sealing lips 32. Three 'or more nozzles 30 could be employed in this arrangement.

FIG. 3B shows an arrangement having an annular nozzle 35 directing a sheet stream of electrolyte inwardly at a wire 36 moving through the center of the annulus. The arrangements of FIGS. 3A and 33 could operate equally well with the wire moving upwardly, downwardly or horizontally, horizontal movement has been found to give be the better results.

FIG. 4A shows an embodiment wherein the strip metal 40 to be treated is moved vertically upwardly through sealing lips 41 passed a slit nozzle orifice 42 formed in the sidewall of a supply header 43. FIG. 4B shows an embodiment wherein strip metal 40 is moved past a plurality of regularly spaced circular nozzle orifices 45 having a common supply header 46. FIG. 4C shows an embodiment wherein the strip 40 is drawn through a quadrant between the vertical and the horizontal, and one or more streams of electrolyte are directed downwardly at an angle to the strip as it passes through the quadrant. The nozzle arrangements of either FIG. 4A or FIG. 48 may be used in this embodiment.

FIG. shows an embodiment wherein a selected area of a length 50 of aconducting strip is delineated by a superimposed mask 51 of insulating material. A stream of electrolyte is directed downwardly onto the strip from a nozzle 52.

FIG. 6 shows an embodiment wherein elongate material 60 to be treated is moved horizontally immediately above the outlet of a short broad nozzle 61. The electrolyte stream flows upwardly and radially outwardly in all directions and flows down through an annular collecting vessel 62 which surrounds the nozzle 61 to a recirculating pump. The nozzle may alternatively be rectangular with its longer direction perpendicular to the direction of motion of the wire, and the collecting vessel comprise two rectangular tanks disposed parallel to the nozzle. This embodiment has been found particularly suitable for the heat treatment of elongate material in strip form.

FIG. 7 shows an electrolyte bath 1 having a stainless steel anode 111 and containing a twice normal aqueous solution of sodium carbonate. A quenching bath 112 containing water is arranged adjacent the bath 110. A cover 113 extends over the bath 110 and has a depending sealing portion 114 dipping beneath the level of the water surface in bath 112 to seal the space above the electrolyte from atmosphere. The enclosed space above the electrolyte contains a protective atmosphere, such as nitrogen or hydrogen.

Wire to be annealed is fed from stock over a spring mounted jockey roll 115, around a contact roll 116, through a seal 117 in the base of bath 110 vertically up through the electrolyte, and round a second contact roll 118. The wire then passes down to the quenching bath 112, round a roll 119, and out over a roll 120 to the drawing and coiling machinery (not shown).

The contact rolls 116 and 118 are connected to electrical earth, whereby the wire drawn over these rolls is earthed. The anode 111 is supplied with DC power and a current density of at least 5 amps per square centimeter is employed. The speed of travel of the wire in relation to the distance it travels through the quenching bath is suitably such that the wires at a temperature of about 200 C. upon exit from the bath, since it can be readily coiled at that temperature. Alternatively, the exit temperature may be arranged to above 300 C. for over ova-raging.

Examples of suitable electrolytes for use in the above described arrangements according to the invention are aqueous solutions of HCl. NaOH, Na- CO KCI. or K CO However, aqueous solutions of other acids, alkalies or salts may alternatively be used. Aqueous solutions of salts of the alkali metals, which are located at the start of the electrochemical series, evolve a copious supply of hydrogen when used as the electrolyte, and are thus particularly suitable. For solutions of salts of metals located more closely above hydrogen in the electrochemical series, the formation of a'stable sheath normally requires higher voltages and current densities. Furthermore, the metal or hydroxide of the metal in solution may be deposited on the cathode.

The minimum electrolyte concentration normally found necessary to permit formation of the sheath is about 0.1 N. However a concentration of about twice' normal, or more, is to be preferred. Increases in the temperature or concentration of the electrolyte generally intensify the electrochemical heating. The presence of oxygen in or above the'electrolyte can also serve to intensify the heating.

Examples of particular electrochemical heat treatment carried out on stainless steel and high carbon content steel wires will now be given by way of example only.

Stainless steel wire one-sixteenth of an inch inch in diameter was annealed using apparatus similar to that shown in FIG. 2B, the anode being of mild steel, the nozzle orifice measuring l /z inches by three-eighths of an inch and the sidewalls being onehalf inch deep. The electrolyte was warm 35 percent K CO solution. On applying a voltage between the electrodes, hydrogen was evolved at the earthed wire (cathode) and oxygen at the nozzle (anode), these gases being swept away with the electrolyte flow. When the voltage was increased 21 volts and the current to 45 amps, a sheath appeared around the wire, the voltage then increasing to 70 volts while the current decreased to 23 amps. The temperature of the wire 'thenrose and upon increasing the voltage further, the wire became red hot.

At an electrolyte temperature of 44 C. and a flow of 3% gallons per minute, with a wire speed of 12 feet per minute, a voltage of 78 volts, and a current of 25 amps, the wire temperature was 950 Czand the heating efficiency about 28 percent. At an electrolyte temperature of 45 C a flow of 4 gallons per minute, a wirespeed of 30 feet per minute a voltage of volts, a current of 49 amps, the wire temperature was 1030 C. at an efficiency of 32 percent.

If the same wire was run three-fourths of an inch below the nozzle, an arc struck and the sheath stabilized at volts and 36 amps. At an electrolyte temperature of 42 C., a flow rate of 4 gallons per minute, a wire speed'of 12 feet per minute, a voltage of volts and a current of 28amp's, the wire temperature was 950 C., andthe efficiency only about 15 percent.

Comparison of the properties of an electrochemically heat treated stainless steel wire (heated to about I050 C.) with a conventionally annealed stainless steel wire, appeared-to indicate that the electrochemically heated wire has a higher ultimate tensile strength, a higher yield to ultimate tensile strength ratio and a slightly lower extension.

A high carbon steel wire 0.048 inches in diameter was treated at 12 feet per minute .in an electrolyte flow at 50 C., and 70 volts, 2l amps raised the temperature of "the wire to I00O C. with an efficiency of about'25 percent.

It will be appreciated, that'the speed with which a given wire can be heat treated by the rnethodof the present invention de pends on a number of factors, in particular the maximum temperature required and the values of voltage and current available. Drawing speeds of 100 feet per minute or more can be obtained under suitable conditions. The velocity of the electrolyte flow for obtaining good results also depends on a number of factors, in particular the speed withwhich a given wire is to be treated. Thus in one case I50 feet per minute was found satisfactory for continuous annealing of steel wire drawn at 30 feet per minute, while a velocity of 80 feet per minute was satisfactory for wire drawn at l5 feet per minute. It would appear that 50 feet per minute may be taken as a practical minimum electrolyte velocity.

It may a be found that a current density of about 5 amps per square centimeter at about 180 volts is sufficient to initiate sheath formation. A practical current density would however normally be at least 25 amps per square centimeter. Similarly in some cases the sheath can be formed at a voltage as low as about 20 volts, though in practice 90 voltsor more might be used.

Where a particular heat treatment requires quenching after the electrochemical heating, the heated material may be drawn through a quenching bath. It may however be preferable to employ other cooling means. In the annealing of stainless steel wire, after electrochemical heating of the wire, the wire can be quenched on a cold pulley. In the heat treatment of high carbon contents steel wire, after electrochemical heating the wire is drawn over a ceramic block and air cooled. The wire is then drawn through a patterning furnace. quenched on a cold pulley and coiled. In one example, at 12 feet per minute, a high carbon wire was heated to l000 C. in 0.625 seconds, air cooled to 800 C. in 2.5 seconds, and within the patterning furnace for 15 seconds.

We claim:

l. A method of heat treating metal wire comprising drawing the wire at a substantially constant speed through a stream of aqueous electrolyte solution so that the immersed length of wire is wholly surrounded by said electrolyte, the wire and the electrolyte forming part of an electrical circuit including a power source, said stream being directed approximately perpendicularly at the moving wire, and cathodically polarizing the wire with respect to an anode associated with the electrolyte, said stream being directed at a flow speed and said wire being polarized at a current density and at a potential sufficient to cause a substantially stable gaseous sheath to form betweenthe electrolyte and the wire wholly surrounding the immersed length of wire and an electrical discharge to take place across the sheath, whereby the wire is rapidly and uniformly heated without damage to the surface thereof.

2. A method according to claim 1 wherein the flow speed of said stream is at least 50 feet per minute.

3. A method according to claim 1 wherein the electrolyte has a concentration of more than 0.1 N.

4. A method according to claim 2 wherein the current density is about 25 amps per square centimeter and the potential difference between an anode and cathode is about volts.

5. A method according to claim 1 wherein the stream is directed downwardly and is confined closely about the wire which is moved substantially horizontally.

6. A method as claimed in claim 1 wherein the stream is directed from a nozzle submerged in a bath of said electrolyte and the wire is drawn through the bath.

7. A method according to claim 1 wherein the wire is quenched after said heat treatment. I 

2. A method according to claim 1 wherein the flow speed of said stream is at least 50 feet per minute.
 3. A method according to claim 1 wherein the electrolyte has a concentration of more than 0.1 N.
 4. A method according to claim 2 wherein the current density is about 25 amps per square centimeter and the potential difference between an anode and cathode is about 90 volts.
 5. A method according to claim 1 wherein the stream is directed downwardly and is confined closely about the wire which is moved substantially horizontally.
 6. A method as claimed in claim 1 wherein the stream is directed from a nozzle submerged in a bath of said electrolyte and the wire is drawn through the bath.
 7. A method according to claim 1 wherein the wire is quenched after said heat treatment. 